Field of the Invention
The invention relates to a device for limiting alternating electric currents.
In order to supply an electric load or device with an alternating electric current, the load is connected to a line branch of an electrical supply network via a switching device. In order to protect the load against excessively high currents, in particular in the event of a short circuit, switching devices having isolating switches which protect the branch and for which melting fuses are generally used, and switching devices having mechanical power switches with switching times of significantly more than one millisecond (1 ms) are employed in the field of low-voltage switching. If a plurality of loads are operated simultaneously in one line branch and if a short circuit occurs at just one of these loads, it is very advantageous if the loads which are not subject to the short circuit can continue to operate without disruption and only the load which is affected by the short circuit is switched off. For this purpose, current-limiting components ("limiters") are necessary directly upstream of each load. These components thus reliably limit the current of the prospective short circuit current to a predefined, noncritical overcurrent value within a time of significantly less than 1 ms and thus before the breaker (of the line protection switch) provided for the line branch is triggered.
In a current-limiting situation, such current limiters must be able to withstand, depending on the network, high voltages of usually up to 700 V and in some cases up to 1200 V which are present at the component. Since the power loss which occurs in the component is very high, it would be particularly advantageous if the current limiter also automatically reduced (intrinsically safe component) the current to values significantly below the predefined overcurrent value while additionally taking up voltage.
The only passive current limiter available on the market is a device which is described in the paper "Polyathylen-Stromwachter fur den Kurzschlu.beta.schutz" ("Polyethylene current monitor for short circuit protection") by T. Hansson, ABB Technik 4/92, pages 35-38 and which has the product name PROLIM. The device is based on a current-dependent conductivity of the grain boundaries of the material employed in this device. However, when the device for current limiting is used frequently, the current saturation value at which the current is limited may change.
Otherwise, in general only active current limiters, in other words current limiters which operate with a control or triggering and which detect the current and limit it through active control when a predefined maximum current value is exceeded, are employed. The German patent document DE 43 30 459 A and the corresponding German utility model DE 94 11 601 U disclose such an active, semiconductor-based current limiter. This active current-limiting semiconductor switch has a first semiconductor region of a predefined conduction type to which in each case one electrode is assigned on surfaces which face away from one another. In the first semiconductor region, further semiconductor regions of the opposite conduction type are disposed spaced apart from one another between the two electrodes. Between the individual further semiconductor regions, channel regions of the first semiconductor region are respectively formed, the channel regions being directed perpendicularly to the two surfaces of the first semiconductor region (vertical channels). A vertical flow of current between the two electrodes is conducted through these channel regions and is thus limited. In order to control the current flow between the two electrodes, a gate voltage, with which the resistance of the channel regions can be controlled, is applied to the oppositely doped semiconductor regions in the first semiconductor region.
German patent application DE 195 48 443 A1, which has been published after two of the priority dates claimed for the instant application and thus cannot be considered as prior art for the respective disclosure of the instant application, discloses a semiconductor configuration having a first semiconductor region of a given conduction type, a contact region which is disposed on a surface of the first semiconductor region, and a second semiconductor region which has the opposite conduction type from the first semiconductor region and is disposed within the first semiconductor region and underneath the contact region. The second semiconductor region extends further in all directions parallel to the surface of the first semiconductor region than the contact region so that in the first semiconductor region at least one channel region is formed. The channel region being bounded in the downward direction by the depletion zone of the p-n junction which is formed between the first semiconductor region and the second semiconductor region, and, in a conductive state, conducting an electric current from the contact region or to the contact region. The at least one channel region is thus disposed laterally in the first semiconductor region and thus has very good saturation properties. On a further surface of the first semiconductor region, which faces away from the aforementioned surface of the first semiconductor region, a further contact region is provided. An operating voltage for the semiconductor region can then be applied between this contact region and the contact region on the other surface of the first semiconductor region.
In one embodiment of the semiconductor configuration described in the published German patent application DE 195 48 443 A1, the channel region is limited, on a side lying opposite the second semiconductor region, by a depletion zone of a further p-n junction, which is formed with the first semiconductor region and at least a third semiconductor region of a conduction type opposite to that of the first semiconductor region. The third semiconductor region is assigned a control electrode for controlling the electrical resistance of the channel region by applying a control voltage. In a further embodiment, the channel region is limited, on a side lying opposite the second semiconductor region, by the depletion zone of at least one Schottky contact. In this embodiment also, a control voltage can be applied to the Schottky contact in order to control the electrical resistance of the channel region.